Flow-drill screws are used to join parts together in one-sided joining operations. Flow-drill screws are threaded fasteners that have a specialized tip that is adapted to penetrate a part by heating the material of the part until the part is locally softened. The flow-drill screw rotates at high speed (i.e. 5,000 rpm) with a substantial axial force (i.e. 1.5 kN) being applied to the part. The material is heated and softened until it is plastically deformable. The rotational speed and axial force are substantially reduced to allow the material to form a threaded sleeve or collar around the flow-drill screw threads on the screw located between the tip and the head of the fastener. The screw is then rotated at a slower speed to tighten the screw into the in-situ formed threaded sleeve, compressing the parts to be joined between the threaded connection and the head of the screw.
One of the advantages of flow drill screws is that they save time and process steps because there is normally no need to provide a hole before inserting the screw and there is no need to tap threads or assemble a nut to a bolt. However, for complex products such as automotive vehicles, airplanes and off-road vehicles that have parts joined by a large number of fasteners, the cycle time (i.e. 2-6 seconds) required to insert a large number of screws may exceed the time available in a manufacturing operation.
In some assembly operations, parts may be secured together with adhesives in addition to flow-drill screws. If adhesive is applied between two panels to be joined, the flow-drill screw operation may be slowed by the insulative effect of the adhesive that reduces the rate of heat transfer from the part on the insertion side of the assembly to the part on the exit side of the assembly. In addition, the heat developed to penetrate the parts may cause the adhesive to foam and may result in degraded adhesive properties.
Some assemblies may include parts that are made of two different types of materials. For example, a polymer part may need to be assembled to an aluminum part. It may be difficult to effectively transfer heat through the polymer part to the second part.
This disclosure is directed to solving the above problems and other problems as summarized below.